The email message can be sent in one of two formats: text or html. Text is just that, ascii text. No formatting at all. Html allows you to change the font (family, size, etc.) in ... html only. There are no predefined buttons to manipulate the size or face, only the font attributes like bold, slanted, strikethrough etc.
To change the font size, search for proper html tags (in one of the thousands of html reference sites).
Then in WebConsole->Tasks->Publish Task->Select your task->E-mail message, click the first button on the second Body toolbar line. The message display switches to html-view.
Add whatever formatting tags you need for changing the font face & size.